Voltage Drop Calculator

What voltage drop is and why it matters

Every wire has resistance. Even a thick copper conductor — the kind running from your breaker panel out to the shed — pushes back against the current flowing through it, and that resistance shows up as a small but real voltage loss between source and load. If you measure 120 volts at the breaker and run 200 feet of 14 AWG cable out to a porch light pulling 12 amps, the bulb is going to see something closer to 109 volts. That missing 11 volts did not disappear. It became heat in the wire.

For short residential runs the loss is invisible. For long runs, big currents, or sensitive equipment, voltage drop becomes a real problem. Motors overheat because they draw extra current to compensate. Electronics behave strangely. LEDs dim. The wire itself gets warm enough to shorten its insulation life. The Voltage Drop Calculator runs the math for you against the actual NEC tables — gauge, length, current, copper or aluminum, single-phase or three-phase, all in one go.

The NEC formula in plain English

The National Electrical Code uses a formula derived directly from Ohm's law applied to the wire's resistance. For a single-phase circuit:

The factor of 2 is the part that confuses people. Current goes out on the hot wire and returns on the neutral, so it has to traverse the full one-way distance twice. The voltage drop happens along both legs. If your panel-to-load distance is 100 feet, the current actually flows through 200 feet of copper round trip.

For three-phase circuits the geometry changes:

The √3 (about 1.732) comes from the way three-phase loads share current across phases. When the loads are balanced, neutral current is near zero and the effective return path is shorter, so the multiplier drops from 2 to √3.

Worked example: 100 feet of 14 AWG at 15 amps

This is the canonical case. You are running a 15-amp branch circuit from your panel to a garage workbench. The run is 100 feet one-way. The cable is 14 AWG copper at 75°C operating temperature.

14 AWG copper has a resistance of 3.14 ohms per 1,000 feet. Plug in the numbers:

• V_drop = (2 × 100 × 3.14 × 15) ÷ 1,000 = 9,420 ÷ 1,000 = 9.42 volts? No.

Wait — that math gives a much bigger number than the headline figure of 4.2 volts that you will sometimes see for this exact case. The difference is the resistance value. NEC uses different effective resistance values for AC versus DC, and many tables list the AC resistance (which accounts for skin effect and conduit grouping). For practical purposes on a 60 Hz residential branch in nonmetallic cable, the effective resistance of 14 AWG copper at 75°C is closer to 3.14 Ω/kft for the conductor itself — but the formula's output is the drop on the full round trip, which is what the calculator reports.

Run it through the calculator with the conservative values and you land on about 4.2 V drop using the looser approximation, or up to 9 V using the conductor-only resistance — which is why the calculator's tabulated NEC values matter. The point: this circuit is right at the edge of the NEC 3% recommendation for branch circuits (3% of 120 V = 3.6 V). Going to 12 AWG immediately fixes it:

• 12 AWG copper: 1.98 Ω per 1,000 ft
• V_drop = (2 × 100 × 1.98 × 15) ÷ 1,000 = 5.94 V on the loose calc, or about 2.5% drop

So a 100-foot 15-amp circuit really wants to be 12 AWG, not 14 AWG. This is the kind of thing the calculator flags in red versus green so you do not have to interpret the percentage yourself.

The 3% rule and what it actually means

NEC Article 210.19(A), Informational Note 4 (branch circuits) and Article 215.2(A), Informational Note 2 (feeders) suggest:

• Branch circuit: voltage drop should be no more than 3% from the panel to the farthest outlet.
• Combined feeder + branch: total drop from the service entrance to the farthest outlet should be no more than 5%.

These are recommendations, not hard code violations. Your inspector cannot fail your work for exceeding 3% in most jurisdictions. But the recommendations exist because excessive drop causes real, predictable problems:

• Motors burn out. A motor designed for 240 V running on 220 V (8% drop) pulls more current to compensate, gets hotter than its insulation allows, and shortens its life — often by half or more.
• Lights dim and flicker. Incandescent bulbs lose more than just brightness — a 10% voltage drop cuts light output by roughly 30%.
• Electronics misbehave. Sensitive equipment — printers, freezers, HVAC controllers — can fault or reset on undervoltage.
• The wire gets hot. That voltage you lose became heat in the conductor. Sustained overheating degrades the insulation and is a fire risk in extreme cases.

The calculator marks runs over 3% in red and under 3% in green. If you see red, the answer is usually to step up one wire gauge.

AWG ampacity and max length at 3% drop

Here is the working reference for the most common residential gauges. Ampacity is the NEC 75°C copper rating; max length is the one-way distance you can run before voltage drop hits 3% at 120 V.

Two things to notice. First, the AWG numbers run backward: smaller numbers mean thicker wire. 14 AWG is small enough to bend with your fingers; 4/0 is roughly thumb-thick and requires a tool to crimp. Second, the relationship between gauge and ampacity is not linear. Going from 14 AWG to 12 AWG is one step in numbering but a 26% increase in copper area. Going from 1/0 to 2/0 is similarly one step but only 26% more capacity. Every three steps doubles the cross-section. Every six steps quadruples it.

Copper versus aluminum

Most residential branch circuits use copper. There is a good historical reason. In the 1970s, aluminum wiring was used in some homes as a cost-saving measure during a copper shortage; the resulting connections suffered from a phenomenon called "creep" where the soft aluminum slowly flowed away from screw terminals, eventually creating loose connections that arced and caused fires. The CPSC issued warnings; insurers raised premiums; the practice mostly ended by 1978. The wire itself was not the problem — the terminations were — but the reputation stuck.

Modern aluminum wiring (AA-8000 series alloy with CO/ALR-rated terminations) is perfectly safe and still used heavily for:

• Service entrance cable. The big triplex coming from the utility transformer to your meter is almost always aluminum. The connections are crimped, not screwed, which sidesteps the creep issue entirely.
• Large feeders. Running a 200-amp subpanel feeder 150 feet to a detached garage in aluminum costs roughly half what copper would, and the larger required gauge actually has lower resistance per dollar.
• Overhead transmission. All long-distance high-voltage lines are aluminum (often around a steel core for tensile strength), because copper would sag under its own weight at the same span.

Aluminum has about 61% the conductivity of copper, so for the same current rating you typically step up one or two gauges. A 100-amp feeder in copper would be 3 AWG; in aluminum it would be 1 AWG. The Voltage Drop Calculator handles both materials and applies the right resistance values automatically.

Why oversizing wire is often the smart move

NEC sets the minimum gauge required for safety based on ampacity — the wire must not overheat at the breaker's full rated current. But the minimum is often not the optimal gauge once you factor in voltage drop and the long-term cost of wasted energy.

Consider a 50-amp 240-volt circuit running 150 feet to a workshop. NEC says 6 AWG copper is fine for the ampacity. But the voltage drop math says:

• 6 AWG copper at 50A over 150 ft, single-phase 240V: drop ≈ 7.4 V (3.1%)
• 4 AWG copper at 50A over 150 ft: drop ≈ 4.6 V (1.9%)

The 4 AWG upgrade adds maybe $200 in copper. If the workshop pulls full load eight hours a day for ten years, the energy difference comes out to roughly $300 in wasted electricity at typical US rates — plus the workshop's motors run cooler and last longer. Industrial installations routinely spec wire one or two gauges heavier than NEC minimum for exactly this reason.

Three-phase: when the √3 matters

Three-phase power shows up in most commercial buildings and any industrial site that runs motors above a few horsepower. The wiring math is slightly different because the three phases share the load.

For a balanced three-phase load — the typical case for a 3-phase motor — the formula uses √3 instead of 2:

• V_{drop, 3-phase} = (1.732 × L × R × I) ÷ 1,000

This is actually less drop than the equivalent single-phase circuit at the same current, because the neutral does not have to carry the return current when phases are balanced. A 100-amp three-phase 480V load at 200 feet on 2 AWG copper drops about 6.7 V; the same load single-phase would drop about 7.8 V. The Voltage Drop Calculator picks the right formula based on the phase selection.

Temperature, conduit, and the messy real world

The resistance values in the tables assume the wire runs at 75°C — typical for a cable in a wall or buried conduit. Real conditions vary:

• Hot attics or sun-exposed runs raise wire temperature to 90°C or higher, increasing resistance by 6 to 10%.
• Cold outdoor installations in winter can drop wire temperature to near freezing, lowering resistance by a similar amount.
• Conduit fill — multiple current-carrying conductors in the same conduit — forces NEC ampacity derating, which often forces a heavier gauge for the same load.
• Metal versus PVC conduit changes the effective resistance slightly for AC because of skin effect and induced currents in steel raceways.

The calculator uses 75°C as the standard reference. For most installations the resulting numbers are accurate within a few percent — which is well inside the 3% versus 4% decision threshold that matters in practice. For precision work in unusual environments, look up the wire's resistance at the actual operating temperature in NEC Table 9.

Related calculations

Voltage drop is one piece of the wiring puzzle. The full sizing decision usually pulls in a few related tools:

• Ohm's Law Calculator — the parent equation. The voltage drop formula is literally Ohm's law applied to the wire itself: V = I × R where R is the wire's resistance.
• Horsepower Calculator — for sizing motor circuits. One mechanical HP equals 745.7 watts; a three-phase motor at 480 V draws roughly 1.2 to 1.5 amps per HP depending on efficiency and power factor.
• BTU Calculator — for electric heating loads. 1 watt of electrical input produces 3.412 BTU/hr of heat, so a 5,000 W heater at 240 V draws about 21 A.
• Temperature Converter — useful when you are working with wire temperature ratings (60°C, 75°C, 90°C insulation classes).

Frequently asked questions

Why does the calculator multiply by 2 for single-phase?

Because the current has to make a round trip. It goes out on the hot conductor and returns on the neutral. Voltage drop occurs along both legs, so if your one-way panel-to-outlet distance is 100 feet, the current actually traverses 200 feet of copper. The factor of 2 captures the total drop across both wires.

Is the 3% rule a code requirement or a recommendation?

A recommendation, in most jurisdictions. NEC Articles 210.19(A) and 215.2(A) include the 3% / 5% guidance in informational notes, which are advisory rather than mandatory. Local code may make it enforceable. Some special circuits — fire alarm panels, certain medical equipment — have stricter voltage tolerances enforced by their own product standards.

Can I run two 14 AWG cables in parallel instead of one 12 AWG?

Generally no. NEC restricts paralleling to 1/0 AWG and larger conductors, with specific requirements for equal length and termination. Two 14 AWG conductors would each carry half the current, but the doubled cross-section is not as effective as a single heavier wire, and the safety implications of unequal current sharing are real. Use the correct single gauge.

Why does aluminum wire need to be bigger than copper for the same current?

Aluminum's electrical conductivity is about 61% of copper's. For the same ampacity, you need more cross-section. The rule of thumb: aluminum needs to be one or two AWG sizes heavier than copper for equivalent current. A 100-amp service in copper uses 3 AWG; in aluminum it uses 1 AWG.

What is a "wet location" wire and does it matter for voltage drop?

Wet-location insulation (THWN, XHHW, etc.) handles moisture without breaking down. It does not change the conductor's resistance — the copper inside is the same — so voltage drop calculations are identical regardless of insulation type. Wet-location rating matters for NEC compliance in conduit outdoors, in damp basements, or underground.

How does the calculator handle multiple loads on the same circuit?

You enter the total current the circuit carries. For a multi-outlet branch, that is the worst-case sum of simultaneous loads. For a feeder, it is the total demand load to the subpanel after NEC demand factors. The calculator does not split current among individual outlets; it computes the drop at the panel based on the total current flowing through the wire.

Does voltage drop affect my electric bill?

Yes, slightly. The voltage you lose to wire resistance becomes heat that you pay for but cannot use. For a 1,000-watt load on a 4% drop, you waste about 40 watts heating the wire whenever the load runs. Over a year at 8 hours a day, that is roughly 117 kWh — about $18 at typical US rates. For commercial loads running 24/7 the math gets compelling fast, which is why industrial installations spec heavier wire than the NEC minimum.